CLONE(2)                  (2020-11-01)                   CLONE(2)

     NAME
          clone, __clone2, clone3 - create a child process

     SYNOPSIS
          /* Prototype for the glibc wrapper function */

          #define _GNU_SOURCE
          #include <sched.h>

          int clone(int (*fn)(void *), void *stack, int flags
                    /* pid_t *parent_tid, void *tls, pid_t *child_tid

          /* For the prototype of the raw clone() system call, see NOTES */

          long clone3(struct clone_args *cl_args, size_t size);

          Note: There is not yet a glibc wrapper for clone3(); see
          NOTES.

     DESCRIPTION
          These system calls create a new ("child") process, in a man-
          ner similar to fork(2).

          By contrast with fork(2), these system calls provide more
          precise control over what pieces of execution context are
          shared between the calling process and the child process.
          For example, using these system calls, the caller can con-
          trol whether or not the two processes share the virtual
          address space, the table of file descriptors, and the table
          of signal handlers.  These system calls also allow the new
          child process to be placed in separate namespaces(7).

          Note that in this manual page, "calling process" normally
          corresponds to "parent process".  But see the descriptions
          of CLONE_PARENT and CLONE_THREAD below.

          This page describes the following interfaces:

          *  The glibc clone() wrapper function and the underlying
             system call on which it is based.  The main text
             describes the wrapper function; the differences for the
             raw system call are described toward the end of this
             page.

          *  The newer clone3() system call.

          In the remainder of this page, the terminology "the clone
          call" is used when noting details that apply to all of these
          interfaces,

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        The clone() wrapper function
          When the child process is created with the clone() wrapper
          function, it commences execution by calling the function
          pointed to by the argument fn. (This differs from fork(2),
          where execution continues in the child from the point of the
          fork(2) call.)  The arg argument is passed as the argument
          of the function fn.

          When the fn(arg) function returns, the child process termi-
          nates.  The integer returned by fn is the exit status for
          the child process.  The child process may also terminate
          explicitly by calling exit(2) or after receiving a fatal
          signal.

          The stack argument specifies the location of the stack used
          by the child process.  Since the child and calling process
          may share memory, it is not possible for the child process
          to execute in the same stack as the calling process.  The
          calling process must therefore set up memory space for the
          child stack and pass a pointer to this space to clone().
          Stacks grow downward on all processors that run Linux
          (except the HP PA processors), so stack usually points to
          the topmost address of the memory space set up for the child
          stack.  Note that clone() does not provide a means whereby
          the caller can inform the kernel of the size of the stack
          area.

          The remaining arguments to clone() are discussed below.

        clone3()
          The clone3() system call provides a superset of the func-
          tionality of the older clone() interface.  It also provides
          a number of API improvements, including: space for addi-
          tional flags bits; cleaner separation in the use of various
          arguments; and the ability to specify the size of the
          child's stack area.

          As with fork(2), clone3() returns in both the parent and the
          child.  It returns 0 in the child process and returns the
          PID of the child in the parent.

          The cl_args argument of clone3() is a structure of the fol-
          lowing form:

              struct clone_args {
                  u64 flags;        /* Flags bit mask */
                  u64 pidfd;        /* Where to store PID file descriptor
                                       (pid_t *) */
                  u64 child_tid;    /* Where to store child TID,
                                       in childaqs memory (pid_t *) */
                  u64 parent_tid;   /* Where to store child TID,
                                       in parentaqs memory (int *) */

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                  u64 exit_signal;  /* Signal to deliver to parent on
                                       child termination */
                  u64 stack;        /* Pointer to lowest byte of stack */
                  u64 stack_size;   /* Size of stack */
                  u64 tls;          /* Location of new TLS */
                  u64 set_tid;      /* Pointer to a pid_t array
                                       (since Linux 5.5) */
                  u64 set_tid_size; /* Number of elements in set_tid
                                       (since Linux 5.5) */
                  u64 cgroup;       /* File descriptor for target cgroup
                                       of child (since Linux 5.7) */
              };

          The size argument that is supplied to clone3() should be
          initialized to the size of this structure.  (The existence
          of the size argument permits future extensions to the
          clone_args structure.)

          The stack for the child process is specified via
          cl_args.stack, which points to the lowest byte of the stack
          area, and cl_args.stack_size, which specifies the size of
          the stack in bytes.  In the case where the CLONE_VM flag
          (see below) is specified, a stack must be explicitly allo-
          cated and specified.  Otherwise, these two fields can be
          specified as NULL and 0, which causes the child to use the
          same stack area as the parent (in the child's own virtual
          address space).

          The remaining fields in the cl_args argument are discussed
          below.

        Equivalence between clone() and clone3() arguments
          Unlike the older clone() interface, where arguments are
          passed individually, in the newer clone3() interface the
          arguments are packaged into the clone_args structure shown
          above.  This structure allows for a superset of the informa-
          tion passed via the clone() arguments.

          The following table shows the equivalence between the argu-
          ments of clone() and the fields in the clone_args argument
          supplied to clone3():
               lb lb lb l l l li li l.  clone()   clone3()  Notes
                    cl_args field flags & ti0xff  flags     For most
               flags; details below parent_tid     pidfd     See
               CLONE_PIDFD child_tid child_tid See CLONE_CHILD_SETTID
               parent_tid     parent_tid     See CLONE_PARENT_SETTID
               flags & 0xff   exit_signal stack     stack ---
                 stack_size tls  tls  See CLONE_SETTLS ---
                 set_tid   See below for details ---  set_tid_size --
               -  cgroup    See CLONE_INTO_CGROUP

        The child termination signal

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          When the child process terminates, a signal may be sent to
          the parent.  The termination signal is specified in the low
          byte of flags (clone()) or in cl_args.exit_signal
          (clone3()).  If this signal is specified as anything other
          than SIGCHLD, then the parent process must specify the
          __WALL or __WCLONE options when waiting for the child with
          wait(2).  If no signal (i.e., zero) is specified, then the
          parent process is not signaled when the child terminates.

        The set_tid array
          By default, the kernel chooses the next sequential PID for
          the new process in each of the PID namespaces where it is
          present.  When creating a process with clone3(), the set_tid
          array (available since Linux 5.5) can be used to select spe-
          cific PIDs for the process in some or all of the PID names-
          paces where it is present.  If the PID of the newly created
          process should be set only for the current PID namespace or
          in the newly created PID namespace (if flags contains
          CLONE_NEWPID) then the first element in the set_tid array
          has to be the desired PID and set_tid_size needs to be 1.

          If the PID of the newly created process should have a cer-
          tain value in multiple PID namespaces, then the set_tid
          array can have multiple entries.  The first entry defines
          the PID in the most deeply nested PID namespace and each of
          the following entries contains the PID in the corresponding
          ancestor PID namespace.  The number of PID namespaces in
          which a PID should be set is defined by set_tid_size which
          cannot be larger than the number of currently nested PID
          namespaces.

          To create a process with the following PIDs in a PID names-
          pace hierarchy:
               lb lb lb l l l.  PID NS level   Requested PID  Notes
               0    31496     Outermost PID namespace 1    42
               2    7    Innermost PID namespace

          Set the array to:

              set_tid[0] = 7;
              set_tid[1] = 42;
              set_tid[2] = 31496;
              set_tid_size = 3;

          If only the PIDs in the two innermost PID namespaces need to
          be specified, set the array to:

              set_tid[0] = 7;
              set_tid[1] = 42;
              set_tid_size = 2;

          The PID in the PID namespaces outside the two innermost PID

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          namespaces will be selected the same way as any other PID is
          selected.

          The set_tid feature requires CAP_SYS_ADMIN or (since Linux
          5.9) CAP_CHECKPOINT_RESTORE in all owning user namespaces of
          the target PID namespaces.

          Callers may only choose a PID greater than 1 in a given PID
          namespace if an init process (i.e., a process with PID 1)
          already exists in that namespace.  Otherwise the PID entry
          for this PID namespace must be 1.

        The flags mask
          Both clone() and clone3() allow a flags bit mask that modi-
          fies their behavior and allows the caller to specify what is
          shared between the calling process and the child process.
          This bit mask-the flags argument of clone() or the
          cl_args.flags field passed to clone3()-is referred to as the
          flags mask in the remainder of this page.

          The flags mask is specified as a bitwise-OR of zero or more
          of the constants listed below.  Except as noted below, these
          flags are available (and have the same effect) in both
          clone() and clone3().

          CLONE_CHILD_CLEARTID (since Linux 2.5.49)
               Clear (zero) the child thread ID at the location
               pointed to by child_tid (clone()) or cl_args.child_tid
               (clone3()) in child memory when the child exits, and do
               a wakeup on the futex at that address.  The address
               involved may be changed by the set_tid_address(2) sys-
               tem call.  This is used by threading libraries.

          CLONE_CHILD_SETTID (since Linux 2.5.49)
               Store the child thread ID at the location pointed to by
               child_tid (clone()) or cl_args.child_tid (clone3()) in
               the child's memory.  The store operation completes
               before the clone call returns control to user space in
               the child process.  (Note that the store operation may
               not have completed before the clone call returns in the
               parent process, which will be relevant if the CLONE_VM
               flag is also employed.)

          CLONE_CLEAR_SIGHAND (since Linux 5.5)
               By default, signal dispositions in the child thread are
               the same as in the parent.  If this flag is specified,
               then all signals that are handled in the parent are
               reset to their default dispositions (SIG_DFL) in the
               child.

               Specifying this flag together with CLONE_SIGHAND is
               nonsensical and disallowed.

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          CLONE_DETACHED (historical)
               For a while (during the Linux 2.5 development series)
               there was a CLONE_DETACHED flag, which caused the par-
               ent not to receive a signal when the child terminated.
               Ultimately, the effect of this flag was subsumed under
               the CLONE_THREAD flag and by the time Linux 2.6.0 was
               released, this flag had no effect.  Starting in Linux
               2.6.2, the need to give this flag together with
               CLONE_THREAD disappeared.

               This flag is still defined, but it is usually ignored
               when calling clone().  However, see the description of
               CLONE_PIDFD for some exceptions.

          CLONE_FILES (since Linux 2.0)
               If CLONE_FILES is set, the calling process and the
               child process share the same file descriptor table.
               Any file descriptor created by the calling process or
               by the child process is also valid in the other pro-
               cess.  Similarly, if one of the processes closes a file
               descriptor, or changes its associated flags (using the
               fcntl(2) F_SETFD operation), the other process is also
               affected.  If a process sharing a file descriptor table
               calls execve(2), its file descriptor table is dupli-
               cated (unshared).

               If CLONE_FILES is not set, the child process inherits a
               copy of all file descriptors opened in the calling pro-
               cess at the time of the clone call.  Subsequent opera-
               tions that open or close file descriptors, or change
               file descriptor flags, performed by either the calling
               process or the child process do not affect the other
               process.  Note, however, that the duplicated file
               descriptors in the child refer to the same open file
               descriptions as the corresponding file descriptors in
               the calling process, and thus share file offsets and
               file status flags (see open(2)).

          CLONE_FS (since Linux 2.0)
               If CLONE_FS is set, the caller and the child process
               share the same filesystem information.  This includes
               the root of the filesystem, the current working direc-
               tory, and the umask.  Any call to chroot(2), chdir(2),
               or umask(2) performed by the calling process or the
               child process also affects the other process.

               If CLONE_FS is not set, the child process works on a
               copy of the filesystem information of the calling pro-
               cess at the time of the clone call.  Calls to
               chroot(2), chdir(2), or umask(2) performed later by one
               of the processes do not affect the other process.

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          CLONE_INTO_CGROUP (since Linux 5.7)
               By default, a child process is placed in the same ver-
               sion 2 cgroup as its parent.  The CLONE_INTO_CGROUP
               flag allows the child process to be created in a dif-
               ferent version 2 cgroup.  (Note that CLONE_INTO_CGROUP
               has effect only for version 2 cgroups.)

               In order to place the child process in a different
               cgroup, the caller specifies CLONE_INTO_CGROUP in
               cl_args.flags and passes a file descriptor that refers
               to a version 2 cgroup in the cl_args.cgroup field.
               (This file descriptor can be obtained by opening a
               cgroup v2 directory using either the O_RDONLY or the
               O_PATH flag.)  Note that all of the usual restrictions
               (described in cgroups(7)) on placing a process into a
               version 2 cgroup apply.

               Among the possible use cases for CLONE_INTO_CGROUP are
               the following:

               *  Spawning a process into a cgroup different from the
                  parent's cgroup makes it possible for a service man-
                  ager to directly spawn new services into dedicated
                  cgroups.  This eliminates the accounting jitter that
                  would be caused if the child process was first cre-
                  ated in the same cgroup as the parent and then moved
                  into the target cgroup.  Furthermore, spawning the
                  child process directly into a target cgroup is sig-
                  nificantly cheaper than moving the child process
                  into the target cgroup after it has been created.

               *  The CLONE_INTO_CGROUP flag also allows the creation
                  of frozen child processes by spawning them into a
                  frozen cgroup.  (See cgroups(7) for a description of
                  the freezer controller.)

               *  For threaded applications (or even thread implemen-
                  tations which make use of cgroups to limit individ-
                  ual threads), it is possible to establish a fixed
                  cgroup layout before spawning each thread directly
                  into its target cgroup.

          CLONE_IO (since Linux 2.6.25)
               If CLONE_IO is set, then the new process shares an I/O
               context with the calling process.  If this flag is not
               set, then (as with fork(2)) the new process has its own
               I/O context.

               The I/O context is the I/O scope of the disk scheduler
               (i.e., what the I/O scheduler uses to model scheduling
               of a process's I/O).  If processes share the same I/O
               context, they are treated as one by the I/O scheduler.

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               As a consequence, they get to share disk time.  For
               some I/O schedulers, if two processes share an I/O con-
               text, they will be allowed to interleave their disk
               access.  If several threads are doing I/O on behalf of
               the same process (aio_read(3), for instance), they
               should employ CLONE_IO to get better I/O performance.

               If the kernel is not configured with the CONFIG_BLOCK
               option, this flag is a no-op.

          CLONE_NEWCGROUP (since Linux 4.6)
               Create the process in a new cgroup namespace.  If this
               flag is not set, then (as with fork(2)) the process is
               created in the same cgroup namespaces as the calling
               process.

               For further information on cgroup namespaces, see
               cgroup_namespaces(7).

               Only a privileged process (CAP_SYS_ADMIN) can employ
               CLONE_NEWCGROUP.

          CLONE_NEWIPC (since Linux 2.6.19)
               If CLONE_NEWIPC is set, then create the process in a
               new IPC namespace.  If this flag is not set, then (as
               with fork(2)), the process is created in the same IPC
               namespace as the calling process.

               For further information on IPC namespaces, see
               ipc_namespaces(7).

               Only a privileged process (CAP_SYS_ADMIN) can employ
               CLONE_NEWIPC.  This flag can't be specified in conjunc-
               tion with CLONE_SYSVSEM.

          CLONE_NEWNET (since Linux 2.6.24)
               (The implementation of this flag was completed only by
               about kernel version 2.6.29.)

               If CLONE_NEWNET is set, then create the process in a
               new network namespace.  If this flag is not set, then
               (as with fork(2)) the process is created in the same
               network namespace as the calling process.

               For further information on network namespaces, see
               network_namespaces(7).

               Only a privileged process (CAP_SYS_ADMIN) can employ
               CLONE_NEWNET.

          CLONE_NEWNS (since Linux 2.4.19)
               If CLONE_NEWNS is set, the cloned child is started in a

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               new mount namespace, initialized with a copy of the
               namespace of the parent.  If CLONE_NEWNS is not set,
               the child lives in the same mount namespace as the par-
               ent.

               For further information on mount namespaces, see
               namespaces(7) and mount_namespaces(7).

               Only a privileged process (CAP_SYS_ADMIN) can employ
               CLONE_NEWNS.  It is not permitted to specify both
               CLONE_NEWNS and CLONE_FS in the same clone call.

          CLONE_NEWPID (since Linux 2.6.24)
               If CLONE_NEWPID is set, then create the process in a
               new PID namespace.  If this flag is not set, then (as
               with fork(2)) the process is created in the same PID
               namespace as the calling process.

               For further information on PID namespaces, see
               namespaces(7) and pid_namespaces(7).

               Only a privileged process (CAP_SYS_ADMIN) can employ
               CLONE_NEWPID.  This flag can't be specified in conjunc-
               tion with CLONE_THREAD or CLONE_PARENT.

          CLONE_NEWUSER
               (This flag first became meaningful for clone() in Linux
               2.6.23, the current clone() semantics were merged in
               Linux 3.5, and the final pieces to make the user names-
               paces completely usable were merged in Linux 3.8.)

               If CLONE_NEWUSER is set, then create the process in a
               new user namespace.  If this flag is not set, then (as
               with fork(2)) the process is created in the same user
               namespace as the calling process.

               For further information on user namespaces, see
               namespaces(7) and user_namespaces(7).

               Before Linux 3.8, use of CLONE_NEWUSER required that
               the caller have three capabilities: CAP_SYS_ADMIN,
               CAP_SETUID, and CAP_SETGID.  Starting with Linux 3.8,
               no privileges are needed to create a user namespace.

               This flag can't be specified in conjunction with
               CLONE_THREAD or CLONE_PARENT.  For security reasons,
               CLONE_NEWUSER cannot be specified in conjunction with
               CLONE_FS.

          CLONE_NEWUTS (since Linux 2.6.19)
               If CLONE_NEWUTS is set, then create the process in a
               new UTS namespace, whose identifiers are initialized by

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               duplicating the identifiers from the UTS namespace of
               the calling process.  If this flag is not set, then (as
               with fork(2)) the process is created in the same UTS
               namespace as the calling process.

               For further information on UTS namespaces, see
               uts_namespaces(7).

               Only a privileged process (CAP_SYS_ADMIN) can employ
               CLONE_NEWUTS.

          CLONE_PARENT (since Linux 2.3.12)
               If CLONE_PARENT is set, then the parent of the new
               child (as returned by getppid(2)) will be the same as
               that of the calling process.

               If CLONE_PARENT is not set, then (as with fork(2)) the
               child's parent is the calling process.

               Note that it is the parent process, as returned by
               getppid(2), which is signaled when the child termi-
               nates, so that if CLONE_PARENT is set, then the parent
               of the calling process, rather than the calling process
               itself, will be signaled.

               The CLONE_PARENT flag can't be used in clone calls by
               the global init process (PID 1 in the initial PID
               namespace) and init processes in other PID namespaces.
               This restriction prevents the creation of multi-rooted
               process trees as well as the creation of unreapable
               zombies in the initial PID namespace.

          CLONE_PARENT_SETTID (since Linux 2.5.49)
               Store the child thread ID at the location pointed to by
               parent_tid (clone()) or cl_args.parent_tid (clone3())
               in the parent's memory.  (In Linux 2.5.32-2.5.48 there
               was a flag CLONE_SETTID that did this.)  The store
               operation completes before the clone call returns con-
               trol to user space.

          CLONE_PID (Linux 2.0 to 2.5.15)
               If CLONE_PID is set, the child process is created with
               the same process ID as the calling process.  This is
               good for hacking the system, but otherwise of not much
               use.  From Linux 2.3.21 onward, this flag could be
               specified only by the system boot process (PID 0).  The
               flag disappeared completely from the kernel sources in
               Linux 2.5.16.  Subsequently, the kernel silently
               ignored this bit if it was specified in the flags mask.
               Much later, the same bit was recycled for use as the
               CLONE_PIDFD flag.

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          CLONE_PIDFD (since Linux 5.2)
               If this flag is specified, a PID file descriptor refer-
               ring to the child process is allocated and placed at a
               specified location in the parent's memory.  The close-
               on-exec flag is set on this new file descriptor.  PID
               file descriptors can be used for the purposes described
               in pidfd_open(2).

               *  When using clone3(), the PID file descriptor is
                  placed at the location pointed to by cl_args.pidfd.

               *  When using clone(), the PID file descriptor is
                  placed at the location pointed to by parent_tid.
                  Since the parent_tid argument is used to return the
                  PID file descriptor, CLONE_PIDFD cannot be used with
                  CLONE_PARENT_SETTID when calling clone().

               It is currently not possible to use this flag together
               with CLONE_THREAD. This means that the process identi-
               fied by the PID file descriptor will always be a thread
               group leader.

               If the obsolete CLONE_DETACHED flag is specified along-
               side CLONE_PIDFD when calling clone(), an error is
               returned.  An error also results if CLONE_DETACHED is
               specified when calling clone3().  This error behavior
               ensures that the bit corresponding to CLONE_DETACHED
               can be reused for further PID file descriptor features
               in the future.

          CLONE_PTRACE (since Linux 2.2)
               If CLONE_PTRACE is specified, and the calling process
               is being traced, then trace the child also (see
               ptrace(2)).

          CLONE_SETTLS (since Linux 2.5.32)
               The TLS (Thread Local Storage) descriptor is set to
               tls.

               The interpretation of tls and the resulting effect is
               architecture dependent.  On x86, tls is interpreted as
               a struct user_desc * (see set_thread_area(2)).  On
               x86-64 it is the new value to be set for the %fs base
               register (see the ARCH_SET_FS argument to
               arch_prctl(2)).  On architectures with a dedicated TLS
               register, it is the new value of that register.

               Use of this flag requires detailed knowledge and gener-
               ally it should not be used except in libraries imple-
               menting threading.

          CLONE_SIGHAND (since Linux 2.0)

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               If CLONE_SIGHAND is set, the calling process and the
               child process share the same table of signal handlers.
               If the calling process or child process calls
               sigaction(2) to change the behavior associated with a
               signal, the behavior is changed in the other process as
               well.  However, the calling process and child processes
               still have distinct signal masks and sets of pending
               signals.  So, one of them may block or unblock signals
               using sigprocmask(2) without affecting the other pro-
               cess.

               If CLONE_SIGHAND is not set, the child process inherits
               a copy of the signal handlers of the calling process at
               the time of the clone call.  Calls to sigaction(2) per-
               formed later by one of the processes have no effect on
               the other process.

               Since Linux 2.6.0, the flags mask must also include
               CLONE_VM if CLONE_SIGHAND is specified

          CLONE_STOPPED (since Linux 2.6.0)
               If CLONE_STOPPED is set, then the child is initially
               stopped (as though it was sent a SIGSTOP signal), and
               must be resumed by sending it a SIGCONT signal.

               This flag was deprecated from Linux 2.6.25 onward, and
               was removed altogether in Linux 2.6.38.  Since then,
               the kernel silently ignores it without error.  Starting
               with Linux 4.6, the same bit was reused for the
               CLONE_NEWCGROUP flag.

          CLONE_SYSVSEM (since Linux 2.5.10)
               If CLONE_SYSVSEM is set, then the child and the calling
               process share a single list of System V semaphore
               adjustment (semadj) values (see semop(2)).  In this
               case, the shared list accumulates semadj values across
               all processes sharing the list, and semaphore adjust-
               ments are performed only when the last process that is
               sharing the list terminates (or ceases sharing the list
               using unshare(2)).  If this flag is not set, then the
               child has a separate semadj list that is initially
               empty.

          CLONE_THREAD (since Linux 2.4.0)
               If CLONE_THREAD is set, the child is placed in the same
               thread group as the calling process.  To make the
               remainder of the discussion of CLONE_THREAD more read-
               able, the term "thread" is used to refer to the pro-
               cesses within a thread group.

               Thread groups were a feature added in Linux 2.4 to sup-
               port the POSIX threads notion of a set of threads that

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               share a single PID.  Internally, this shared PID is the
               so-called thread group identifier (TGID) for the thread
               group.  Since Linux 2.4, calls to getpid(2) return the
               TGID of the caller.

               The threads within a group can be distinguished by
               their (system-wide) unique thread IDs (TID).  A new
               thread's TID is available as the function result
               returned to the caller, and a thread can obtain its own
               TID using gettid(2).

               When a clone call is made without specifying
               CLONE_THREAD, then the resulting thread is placed in a
               new thread group whose TGID is the same as the thread's
               TID.  This thread is the leader of the new thread
               group.

               A new thread created with CLONE_THREAD has the same
               parent process as the process that made the clone call
               (i.e., like CLONE_PARENT), so that calls to getppid(2)
               return the same value for all of the threads in a
               thread group.  When a CLONE_THREAD thread terminates,
               the thread that created it is not sent a SIGCHLD (or
               other termination) signal; nor can the status of such a
               thread be obtained using wait(2).  (The thread is said
               to be detached.)

               After all of the threads in a thread group terminate
               the parent process of the thread group is sent a
               SIGCHLD (or other termination) signal.

               If any of the threads in a thread group performs an
               execve(2), then all threads other than the thread group
               leader are terminated, and the new program is executed
               in the thread group leader.

               If one of the threads in a thread group creates a child
               using fork(2), then any thread in the group can wait(2)
               for that child.

               Since Linux 2.5.35, the flags mask must also include
               CLONE_SIGHAND if CLONE_THREAD is specified (and note
               that, since Linux 2.6.0, CLONE_SIGHAND also requires
               CLONE_VM to be included).

               Signal dispositions and actions are process-wide: if an
               unhandled signal is delivered to a thread, then it will
               affect (terminate, stop, continue, be ignored in) all
               members of the thread group.

               Each thread has its own signal mask, as set by
               sigprocmask(2).

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               A signal may be process-directed or thread-directed.  A
               process-directed signal is targeted at a thread group
               (i.e., a TGID), and is delivered to an arbitrarily
               selected thread from among those that are not blocking
               the signal.  A signal may be process-directed because
               it was generated by the kernel for reasons other than a
               hardware exception, or because it was sent using
               kill(2) or sigqueue(3).  A thread-directed signal is
               targeted at (i.e., delivered to) a specific thread.  A
               signal may be thread directed because it was sent using
               tgkill(2) or pthread_sigqueue(3), or because the thread
               executed a machine language instruction that triggered
               a hardware exception (e.g., invalid memory access trig-
               gering SIGSEGV or a floating-point exception triggering
               SIGFPE).

               A call to sigpending(2) returns a signal set that is
               the union of the pending process-directed signals and
               the signals that are pending for the calling thread.

               If a process-directed signal is delivered to a thread
               group, and the thread group has installed a handler for
               the signal, then the handler will be invoked in exactly
               one, arbitrarily selected member of the thread group
               that has not blocked the signal.  If multiple threads
               in a group are waiting to accept the same signal using
               sigwaitinfo(2), the kernel will arbitrarily select one
               of these threads to receive the signal.

          CLONE_UNTRACED (since Linux 2.5.46)
               If CLONE_UNTRACED is specified, then a tracing process
               cannot force CLONE_PTRACE on this child process.

          CLONE_VFORK (since Linux 2.2)
               If CLONE_VFORK is set, the execution of the calling
               process is suspended until the child releases its vir-
               tual memory resources via a call to execve(2) or
               _exit(2) (as with vfork(2)).

               If CLONE_VFORK is not set, then both the calling pro-
               cess and the child are schedulable after the call, and
               an application should not rely on execution occurring
               in any particular order.

          CLONE_VM (since Linux 2.0)
               If CLONE_VM is set, the calling process and the child
               process run in the same memory space.  In particular,
               memory writes performed by the calling process or by
               the child process are also visible in the other pro-
               cess.  Moreover, any memory mapping or unmapping per-
               formed with mmap(2) or munmap(2) by the child or call-
               ing process also affects the other process.

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               If CLONE_VM is not set, the child process runs in a
               separate copy of the memory space of the calling pro-
               cess at the time of the clone call.  Memory writes or
               file mappings/unmappings performed by one of the pro-
               cesses do not affect the other, as with fork(2).

               If the CLONE_VM flag is specified and the CLONE_VM flag
               is not specified, then any alternate signal stack that
               was established by sigaltstack(2) is cleared in the
               child process.

     RETURN VALUE
          On success, the thread ID of the child process is returned
          in the caller's thread of execution.  On failure, -1 is
          returned in the caller's context, no child process will be
          created, and errno will be set appropriately.

     ERRORS
          EAGAIN
               Too many processes are already running; see fork(2).

          EBUSY (clone3() only)
               CLONE_INTO_CGROUP was specified in cl_args.flags, but
               the file descriptor specified in cl_args.cgroup refers
               to a version 2 cgroup in which a domain controller is
               enabled.

          EEXIST (clone3() only)
               One (or more) of the PIDs specified in set_tid already
               exists in the corresponding PID namespace.

          EINVAL
               Both CLONE_SIGHAND and CLONE_CLEAR_SIGHAND were speci-
               fied in the flags mask.

          EINVAL
               CLONE_SIGHAND was specified in the flags mask, but
               CLONE_VM was not.  (Since Linux 2.6.0.)

          EINVAL
               CLONE_THREAD was specified in the flags mask, but
               CLONE_SIGHAND was not.  (Since Linux 2.5.35.)

          EINVAL
               CLONE_THREAD was specified in the flags mask, but the
               current process previously called unshare(2) with the
               CLONE_NEWPID flag or used setns(2) to reassociate
               itself with a PID namespace.

          EINVAL
               Both CLONE_FS and CLONE_NEWNS were specified in the
               flags mask.

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          EINVAL (since Linux 3.9)
               Both CLONE_NEWUSER and CLONE_FS were specified in the
               flags mask.

          EINVAL
               Both CLONE_NEWIPC and CLONE_SYSVSEM were specified in
               the flags mask.

          EINVAL
               One (or both) of CLONE_NEWPID or CLONE_NEWUSER and one
               (or both) of CLONE_THREAD or CLONE_PARENT were speci-
               fied in the flags mask.

          EINVAL (since Linux 2.6.32)
               CLONE_PARENT was specified, and the caller is an init
               process.

          EINVAL
               Returned by the glibc clone() wrapper function when fn
               or stack is specified as NULL.

          EINVAL
               CLONE_NEWIPC was specified in the flags mask, but the
               kernel was not configured with the CONFIG_SYSVIPC and
               CONFIG_IPC_NS options.

          EINVAL
               CLONE_NEWNET was specified in the flags mask, but the
               kernel was not configured with the CONFIG_NET_NS
               option.

          EINVAL
               CLONE_NEWPID was specified in the flags mask, but the
               kernel was not configured with the CONFIG_PID_NS
               option.

          EINVAL
               CLONE_NEWUSER was specified in the flags mask, but the
               kernel was not configured with the CONFIG_USER_NS
               option.

          EINVAL
               CLONE_NEWUTS was specified in the flags mask, but the
               kernel was not configured with the CONFIG_UTS_NS
               option.

          EINVAL
               stack is not aligned to a suitable boundary for this
               architecture.  For example, on aarch64, stack must be a
               multiple of 16.

          EINVAL (clone3() only)

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               CLONE_DETACHED was specified in the flags mask.

          EINVAL (clone() only)
               CLONE_PIDFD was specified together with CLONE_DETACHED
               in the flags mask.

          EINVAL
               CLONE_PIDFD was specified together with CLONE_THREAD in
               the flags mask.

          EINVAL (clone() only)
               CLONE_PIDFD was specified together with
               CLONE_PARENT_SETTID in the flags mask.

          EINVAL (clone3() only)
               set_tid_size is greater than the number of nested PID
               namespaces.

          EINVAL (clone3() only)
               One of the PIDs specified in set_tid was an invalid.

          EINVAL (AArch64 only, Linux 4.6 and earlier)
               stack was not aligned to a 126-bit boundary.

          ENOMEM
               Cannot allocate sufficient memory to allocate a task
               structure for the child, or to copy those parts of the
               caller's context that need to be copied.

          ENOSPC (since Linux 3.7)
               CLONE_NEWPID was specified in the flags mask, but the
               limit on the nesting depth of PID namespaces would have
               been exceeded; see pid_namespaces(7).

          ENOSPC (since Linux 4.9; beforehand EUSERS)
               CLONE_NEWUSER was specified in the flags mask, and the
               call would cause the limit on the number of nested user
               namespaces to be exceeded.  See user_namespaces(7).

               From Linux 3.11 to Linux 4.8, the error diagnosed in
               this case was EUSERS.

          ENOSPC (since Linux 4.9)
               One of the values in the flags mask specified the cre-
               ation of a new user namespace, but doing so would have
               caused the limit defined by the corresponding file in
               /proc/sys/user to be exceeded.  For further details,
               see namespaces(7).

          EOPNOTSUPP (clone3() only)
               CLONE_INTO_CGROUP was specified in cl_args.flags, but
               the file descriptor specified in cl_args.cgroup refers

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               to a version 2 cgroup that is in the domain invalid
               state.

          EPERM
               CLONE_NEWCGROUP, CLONE_NEWIPC, CLONE_NEWNET,
               CLONE_NEWNS, CLONE_NEWPID, or CLONE_NEWUTS was speci-
               fied by an unprivileged process (process without
               CAP_SYS_ADMIN).

          EPERM
               CLONE_PID was specified by a process other than process
               0.  (This error occurs only on Linux 2.5.15 and ear-
               lier.)

          EPERM
               CLONE_NEWUSER was specified in the flags mask, but
               either the effective user ID or the effective group ID
               of the caller does not have a mapping in the parent
               namespace (see user_namespaces(7)).

          EPERM (since Linux 3.9)
               CLONE_NEWUSER was specified in the flags mask and the
               caller is in a chroot environment (i.e., the caller's
               root directory does not match the root directory of the
               mount namespace in which it resides).

          EPERM (clone3() only)
               set_tid_size was greater than zero, and the caller
               lacks the CAP_SYS_ADMIN capability in one or more of
               the user namespaces that own the corresponding PID
               namespaces.

          ERESTARTNOINTR (since Linux 2.6.17)
               System call was interrupted by a signal and will be
               restarted.  (This can be seen only during a trace.)

          EUSERS (Linux 3.11 to Linux 4.8)
               CLONE_NEWUSER was specified in the flags mask, and the
               limit on the number of nested user namespaces would be
               exceeded.  See the discussion of the ENOSPC error
               above.

     VERSIONS
          The clone3() system call first appeared in Linux 5.3.

     CONFORMING TO
          These system calls are Linux-specific and should not be used
          in programs intended to be portable.

     NOTES
          One use of these systems calls is to implement threads: mul-
          tiple flows of control in a program that run concurrently in

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          a shared address space.

          Glibc does not provide a wrapper for clone3(); call it using
          syscall(2).

          Note that the glibc clone() wrapper function makes some
          changes in the memory pointed to by stack (changes required
          to set the stack up correctly for the child) before invoking
          the clone() system call.  So, in cases where clone() is used
          to recursively create children, do not use the buffer
          employed for the parent's stack as the stack of the child.

          The kcmp(2) system call can be used to test whether two pro-
          cesses share various resources such as a file descriptor
          table, System V semaphore undo operations, or a virtual
          address space.

          Handlers registered using pthread_atfork(3) are not executed
          during a clone call.

          In the Linux 2.4.x series, CLONE_THREAD generally does not
          make the parent of the new thread the same as the parent of
          the calling process.  However, for kernel versions 2.4.7 to
          2.4.18 the CLONE_THREAD flag implied the CLONE_PARENT flag
          (as in Linux 2.6.0 and later).

          On i386, clone() should not be called through vsyscall, but
          directly through int $0x80.

        C library/kernel differences
          The raw clone() system call corresponds more closely to
          fork(2) in that execution in the child continues from the
          point of the call.  As such, the fn and arg arguments of the
          clone() wrapper function are omitted.

          In contrast to the glibc wrapper, the raw clone() system
          call accepts NULL as a stack argument (and clone3() likewise
          allows cl_args.stack to be NULL).  In this case, the child
          uses a duplicate of the parent's stack.  (Copy-on-write
          semantics ensure that the child gets separate copies of
          stack pages when either process modifies the stack.)  In
          this case, for correct operation, the CLONE_VM option should
          not be specified.  (If the child shares the parent's memory
          because of the use of the CLONE_VM flag, then no copy-on-
          write duplication occurs and chaos is likely to result.)

          The order of the arguments also differs in the raw system
          call, and there are variations in the arguments across
          architectures, as detailed in the following paragraphs.

          The raw system call interface on x86-64 and some other
          architectures (including sh, tile, and alpha) is:

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              long clone(unsigned long flags, void *stack,
                         int *parent_tid, int *child_tid,
                         unsigned long tls);

          On x86-32, and several other common architectures (including
          score, ARM, ARM 64, PA-RISC, arc, Power PC, xtensa, and
          MIPS), the order of the last two arguments is reversed:

              long clone(unsigned long flags, void *stack,
                        int *parent_tid, unsigned long tls,
                        int *child_tid);

          On the cris and s390 architectures, the order of the first
          two arguments is reversed:

              long clone(void *stack, unsigned long flags,
                         int *parent_tid, int *child_tid,
                         unsigned long tls);

          On the microblaze architecture, an additional argument is
          supplied:

              long clone(unsigned long flags, void *stack,
                         int stack_size,         /* Size of stack */
                         int *parent_tid, int *child_tid,
                         unsigned long tls);

        blackfin, m68k, and sparc
          The argument-passing conventions on blackfin, m68k, and
          sparc are different from the descriptions above.  For
          details, see the kernel (and glibc) source.

        ia64
          On ia64, a different interface is used:

              int __clone2(int (*fn)(void *),
                           void *stack_base, size_t stack_size,
                           int flags, void *arg, ...
                        /* pid_t *parent_tid, struct user_desc *tls,
                           pid_t *child_tid */ );

          The prototype shown above is for the glibc wrapper function;
          for the system call itself, the prototype can be described
          as follows (it is identical to the clone() prototype on
          microblaze):

              long clone2(unsigned long flags, void *stack_base,
                          int stack_size,         /* Size of stack */
                          int *parent_tid, int *child_tid,
                          unsigned long tls);

          __clone2() operates in the same way as clone(), except that

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          stack_base points to the lowest address of the child's stack
          area, and stack_size specifies the size of the stack pointed
          to by stack_base.

        Linux 2.4 and earlier
          In Linux 2.4 and earlier, clone() does not take arguments
          parent_tid, tls, and child_tid.

     BUGS
          GNU C library versions 2.3.4 up to and including 2.24 con-
          tained a wrapper function for getpid(2) that performed cach-
          ing of PIDs.  This caching relied on support in the glibc
          wrapper for clone(), but limitations in the implementation
          meant that the cache was not up to date in some circum-
          stances.  In particular, if a signal was delivered to the
          child immediately after the clone() call, then a call to
          getpid(2) in a handler for the signal could return the PID
          of the calling process ("the parent"), if the clone wrapper
          had not yet had a chance to update the PID cache in the
          child.  (This discussion ignores the case where the child
          was created using CLONE_THREAD, when getpid(2) should return
          the same value in the child and in the process that called
          clone(), since the caller and the child are in the same
          thread group.  The stale-cache problem also does not occur
          if the flags argument includes CLONE_VM.)  To get the truth,
          it was sometimes necessary to use code such as the follow-
          ing:

              #include <syscall.h>

              pid_t mypid;

              mypid = syscall(SYS_getpid);

          Because of the stale-cache problem, as well as other prob-
          lems noted in getpid(2), the PID caching feature was removed
          in glibc 2.25.

     EXAMPLES
          The following program demonstrates the use of clone() to
          create a child process that executes in a separate UTS
          namespace.  The child changes the hostname in its UTS names-
          pace.  Both parent and child then display the system host-
          name, making it possible to see that the hostname differs in
          the UTS namespaces of the parent and child.  For an example
          of the use of this program, see setns(2).

          Within the sample program, we allocate the memory that is to
          be used for the child's stack using mmap(2) rather than
          malloc(3) for the following reasons:

          *  mmap(2) allocates a block of memory that starts on a page

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             boundary and is a multiple of the page size.  This is
             useful if we want to establish a guard page (a page with
             protection PROT_NONE) at the end of the stack using
             mprotect(2).

          *  We can specify the MAP_STACK flag to request a mapping
             that is suitable for a stack.  For the moment, this flag
             is a no-op on Linux, but it exists and has effect on some
             other systems, so we should include it for portability.

        Program source
          #define _GNU_SOURCE
          #include <sys/wait.h>
          #include <sys/utsname.h>
          #include <sched.h>
          #include <string.h>
          #include <stdint.h>
          #include <stdio.h>
          #include <stdlib.h>
          #include <unistd.h>
          #include <sys/mman.h>

          #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                                  } while (0)

          static int              /* Start function for cloned child */
          childFunc(void *arg)
          {
              struct utsname uts;

              /* Change hostname in UTS namespace of child */

              if (sethostname(arg, strlen(arg)) == -1)
                  errExit("sethostname");

              /* Retrieve and display hostname */

              if (uname(&uts) == -1)
                  errExit("uname");
              printf("uts.nodename in child:  %s\n", uts.nodename);

              /* Keep the namespace open for a while, by sleeping.
                 This allows some experimentation--for example, another
                 process might join the namespace. */

              sleep(200);

              return 0;           /* Child terminates now */
          }

          #define STACK_SIZE (1024 * 1024)    /* Stack size for cloned child */

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          int
          main(int argc, char *argv[])
          {
              char *stack;                    /* Start of stack buffer */
              char *stackTop;                 /* End of stack buffer */
              pid_t pid;
              struct utsname uts;

              if (argc < 2) {
                  fprintf(stderr, "Usage: %s <child-hostname>\n", argv[0]);
                  exit(EXIT_SUCCESS);
              }

              /* Allocate memory to be used for the stack of the child */

              stack = mmap(NULL, STACK_SIZE, PROT_READ | PROT_WRITE,
                           MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
              if (stack == MAP_FAILED)
                  errExit("mmap");

              stackTop = stack + STACK_SIZE;  /* Assume stack grows downward */

              /* Create child that has its own UTS namespace;
                 child commences execution in childFunc() */

              pid = clone(childFunc, stackTop, CLONE_NEWUTS | SIGCHLD, argv[1]);
              if (pid == -1)
                  errExit("clone");
              printf("clone() returned %jd\n", (intmax_t) pid);

              /* Parent falls through to here */

              sleep(1);           /* Give child time to change its hostname */

              /* Display hostname in parentaqs UTS namespace. This will be
                 different from hostname in childaqs UTS namespace. */

              if (uname(&uts) == -1)
                  errExit("uname");
              printf("uts.nodename in parent: %s\n", uts.nodename);

              if (waitpid(pid, NULL, 0) == -1)    /* Wait for child */
                  errExit("waitpid");
              printf("child has terminated\n");

              exit(EXIT_SUCCESS);
          }

     SEE ALSO
          fork(2), futex(2), getpid(2), gettid(2), kcmp(2), mmap(2),
          pidfd_open(2), set_thread_area(2), set_tid_address(2),
          setns(2), tkill(2), unshare(2), wait(2), capabilities(7),

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          namespaces(7), pthreads(7)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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